1. Field of the Invention
The present invention relates to a power supply converter of the type having rectifiers connected to an AC source to supply DC power to a number of coupled output stages. The invention is particularly directed to a converter suitable for a gradient amplifier of an NMR-tomography apparatus in which a number of power output stages are switched in series to drive a gradient coil.
2. Description of the Prior Art
German OS 43 04 517 discloses a power supply of this type. Two rectifiers are therein connected to an AC-voltage via respective isolating transformers. The rectifiers are able to convey energy to a respective power output stages via respective DC-terminals. The output terminals of the output stages are switched in series and drive a gradient coil.
In operation the output stages supply a regulated voltage of, for example, several hundred volts, to the gradient coil in order to periodically build up and dismantle a current of 300 A, for example. An exactly prescribed current curve is maintained which has a cycle duration of 20 ms, for example. During a cycle, considerable amounts of energy flow from the output stages to the coil and back again. In normal operation, each of the output stages respectively contributes to balancing the losses arising therein. Thus, in normal operation, energy flows via the rectifier from the AC-voltage source to one of the output stages and then further on to the gradient coil, this flow being averaged over the cycle duration (with no accounting for the blind current).
There are, however, other possible modes of operation, in which one output stage draws energy stored in the gradient coil while another output stage delivers this energy. This can be the case particularly when the output stages are driven differently or have different characteristics. For example, when the prescribed current curve has steep leading edges and flat trailing edges, and the output alternatively stages assume the functions of a constant load output stage and a peak load output stage, it is possible for the peak load output stage to be connected only during the steep leading edges of the current. The energy delivered to the gradient coil the peak load output stage is then fed back exclusively to the constant load output stage during the flatter trailing edges of the current. This can lead to an excessive loading (over-voltage) of a charging capacitor which is arranged in a DC-voltage intermediate circuit formed by the constant load output stage and the allocated rectifier. The power supply can be damaged by the over-voltage.
This kind of undesirable loading is also possible in other situations; for example, when the output stages are driven so that they deliver voltages of differing polarity. Deviations in the electrical characteristics of the output stages can also lead to a load transfer between the output stages even when the driving is the same.
To avoid an over-voltage in the individual DC-voltage intermediate circuits, German OS 43 04 517 teaches the use of discharging resistors as a primary alternative, these resistors being switched parallel to the charging capacitors. Such discharging resistors, however, considerably increase the energy consumption of the power supply and the dissipated heat which must be conducted away by cooling. As a second alternative, German OS 43 04 517 provides teaches the use of a switchable discharging means (DC-DC-converter) for energy distribution between the DC-voltage intermediate circuits. This type of discharging means is costly in terms of circuitry, however, especially when more than two output stages and rectifiers are provided.
German OS 35 39 027 teaches a rotary current rectifier with an electronically driveable switch parallel to each diode. The switch-on phases of the electronically driveable switch are driven synchronously with the conductive phase of the allocated, line-commutated diodes. The rectifier is provided for utilization in the drive circuitry in order to enable a backflow of the braking energy into the network in a braking operation.
European Application 0 224 198 disclosed a power convention with a bridge rectifier circuit with thyristors respectively connected with opposite polarity across each rectifier each in the bridge circuit. The thyristors are driven according to a pulse-width modulation process, in order to obtain a sinusoidal current flow with low harmonic distortion of the higher orders.